The primary research objectives are to assess the thermoneutral zone and the thermoregulatory capacity of man as related to age and sex. Based on successful past experience, a total of 40-50 young (20-30 years) and 40-50 old (60-80 years) men and women will be recruited from the adult physical fitness program conducted by the UNLV Exercise Physiology Laboratory and from various local clubs and organizations. A physiological profile will be established for each volunteer consisting of a resting ECG, an exercise ECG, pulmonary function tests, blood pressure, body fat and physical fitness tests. Each subject will be exposed to 10 different air temperatures (Ta) in a climatic room: 5, 15, 20, 25, 27, 30, 33, 36, 40, and 45C, for a duration of 2-3 hrs. Physiological assessment during exposure to each Ta will include: rate of 02 consumption, heart rate, blood pressure, ECG, rectal temperature, and skin temperatures at seven sites. Calculations of heat balance will be computed for each Ta condition. At Ta above 30C, sweat rate and sweat composition will also be measured. These physiological measurements will permit analysis of the efficiency of the thermoregulatory mechanisms as influenced by sex and age. The thermoneutral zone will also be established for both young and old men and women.